HER2 is a receptor tyrosine kinase of the epidermal growth factor receptor family. Amplification or overexpression of HER2 has been demonstrated in the development and progression of cancers. Herceptin® (trastuzumab) is an anti-HER2 monoclonal antibody approved for treating HER2-positive metastatic breast cancer and HER2-positive gastric cancer (Trastuzumab [Highlights of Prescribing Information]. South San Francisco, Calif.: Genentech, Inc.; 2014). Ertumaxomab is a tri-specific HER2-CD3 antibody with intact Fc-receptor binding (see, for example, Kiewe et al. 2006, Clin Cancer Res, 12(10): 3085-3091). Ertumaxomab is a rat-mouse antibody; therefore, upon administration to humans, a human anti-mouse antibody response and a human anti-rat antibody response are expected. 2502A, the parental antibody of ertumaxomab, has low affinity for HER2 and low avidity (Diermeier-Daucher et al., MAbs, 2012, 4(5): 614-622). There is a need for therapies capable of mediating T cell cytotoxicity in HER2-positive cancers.